1. Field Of The Invention
The present invention relates to photoimageable compositions that comprise a material that contains one or more photoacid labile groups. In preferred aspects, the compositions are applied electrophoretically to conductive substrates.
2. Background Art
Photoimageable compositions include photoresists which are capable of transferring an image onto a substrate such as a printed circuit board or a lithographic plate. These resists can be negative-acting or positive-acting. After a film layer of a photoresist is deposited on a surface, the film is selectively exposed, typically through a photomask, to a source of activating energy such as ultraviolet light. The photomask has areas that are opaque and other areas that are transparent to the ultraviolet light. The pattern on the photomask formed by opaque and transparent areas defines the pattern, such as a circuit, to be transferred to the substrate.
Exposed portions of a negative-acting photoresist become less soluble in a developing solution than unexposed areas as the result of a photochemical reaction, for example, between a photoinitiator and an ethylenically unsaturated resin of the resist composition. This difference in solubility allows for the selective removal of the photoresist and the transfer of the photomask image to the substrate. In the case of positive-acting photoresists, exposure to activating radiation will cause the coating to become more soluble in a developing solution than the unexposed areas. The imaged photoresist may act as a protective coating for subtractively etching a metal substrate, known as print-and-etch, or allow the exposed bare metal surface to be further built up in thickness by electroplating methods, known as print-and-plate. The resist may then be selectively stripped. For print-and-etch processes, the exposed metal on the substrate may be etched to form the desired pattern or circuit on the substrate surface. The historical background, types and operation of conventional photoresists, are generally described in Photoresist Materials and Processes, W. DeForest, McGraw-Hill, 1975, and Coombs, Printed Circuits Handbook, McGraw Hill (3rd Ed. 1988), both incorporated herein by reference.
Electrophoretic deposition of certain photoresist coatings has been reported. See, for example, U.S. Pat. No. 4,592,816, incorporated herein by reference for its disclosure of making and using electrophoretic photoresists. Electrophoretic deposition involves a process of electrophoresis which is the motion of charged particles through a liquid medium under the influence of an applied electrical field. The deposition is conducted in a cell with the conductive substrate to be coated serving as one of the electrodes. Deposition of a positively charged material onto a negatively charged cathode is referred to as cataphoresis while deposition of a negatively charged material onto an anode is referred to as anaphoresis.
Another class of photosensitive resists is liquid type photoresists. The solid components of these compositions are dissolved in an organic solvent and are applied to a substrate surface by means such as dip-coating, roller-coating, spin coating, curtain coating or screen coating. After application of the liquid composition, the coated substrate is typically heated to remove the volatile organic solvent carrier.
More recently, so-called "chemically amplified" positive-tone liquid type resist systems have been reported. More specifically, certain cationic photoinitiators have been used to induce acid-catalyzed cleavage of functional groups of one or more components of the resist. In particular, certain cationic photoinitiators have been used to induce cleavage of certain "blocking" groups pendant from a photoresist binder, or cleavage of certain blocking groups that comprise a photoresist binder backbone. See, for example, U.S. Pat. Nos. 5,075,199; 4,968,581; 4,883,740; 4,810,613 and 4,491,628, and Canadian Patent Application 2,001,384, all of which are incorporated herein by reference for their teaching of the described materials containing acid labile blocking groups, and methods of making and using the same. Such cleavage is reported to create different solubility characteristics in exposed and unexposed areas of the polymer. Upon selective cleavage of the blocking group through exposure of the resist, a polar functional group is said to be provided, for example, carboxyl or imide.